Various methods are known for observing an object by using electromagnetic waves, such as a method of obtaining data corresponding to an visual image by using visible light and a method of observing an invisible inner structure by using X-rays.
Terahertz waves are electromagnetic waves having a frequency range of 100 GHz to 10 THz and a wavelength range of 30 μm to 3 mm, which transmit through plastics, cloth, paper, semiconductor and the like, and have absorption spectra specific to substance. It is therefore possible to perform physicality analysis and inspection, radiographic imaging and the like. Applied techniques are also being developed. Optical terms such as an optical axis, a polarizer, an analyzer and the like may be used also in the terahertz wave wavelength range.
Since non-destructive composition analysis imaging is possible, this is expected as a composition analysis type inner radiographic imaging means as an alternative to a conventional inner radiographic imaging means (X-rays, ultrasounds and the like). Terahertz spectroscopic imaging has been proposed using characteristic THz absorption for measuring objects unable to be detected by conventional X-ray inspection, such as explosive substance (plastic bomb, flammable liquid and the like) in air port baggage inspection against recent terror and crime, prohibited substance (drug, antihypnotic agent and the like) in a sealed letter.
A method has been proposed wherein a pulse terahertz wave and a probe pulse light beam are input to an electrooptical (EO) crystal noncoaxially, an image by the probe pulse light beam is photographed with a digital camera, and a terahertz temporal waveform is measured in a single shot.
A method has also been proposed wherein a pulse terahertz wave and a probe pulse light beam are input to an electrooptical (EO) crystal coaxially. With this method, a plane (pulse plane) coupling peak positions of intensities of the probe pulse light beam is inclined relative to the pulse plane of the terahertz wave. By inclining one pulse plane relative to the other pulse plane, it becomes possible to obtain the same effects as those of the noncoaxial case.